Alberta researchers help find earliest evidence of mobile life on Earth

Sarah O'Donnell, Postmedia News06.28.2012

University of Alberta post-doctoral fellow Ernesto Pecoits examines rocks that are part of the Tacuari formation in Uruguay where he discovered trails from a tiny slug that date back 585 million years. In a paper published in the journal Science, U of A researchers say those trace fossils are the earliest discovery of complex life.

EDMONTON — University of Alberta scientists say they have uncovered the fossilized trails of an ancient slug that proves complex life evolved 30 million years earlier than established by previous discoveries.

The tiny tracks, found in a South American rock formation, date back 585 million years. The fossil find reported Thursday in the journal Science, confirms what scientists had theorized was possible — but not previously proved — allowing for an important addition to the evolutionary chronology.

"We now have the oldest physical evidence that multi-cellular life that could move was around between 600 million and 585 million years ago," said U of A geomicrobiologist Kurt Konhauser, one of the article's seven co-authors.

Before these animals evolved, there were single-celled life such as bacteria and eventually other creatures such as sponges, which could not move.

"Then by 585 million years ago, we have evidence something bigger was in existence," Konhauser said.

Bigger, of course, is a relative term.

The slug-like animal that created these three millimetre wide burrows through the sand would have been tiny, a millimetre wide and three millimetres long, explained Ernesto Pecoits, a post-doctoral geology fellow at the university and the report's lead author.

Pecoits and Natalie Aubet, a PhD student, discovered the burrows in 2007 as they were studying a unique cluster of rocks known as the Tacuari formation in eastern Uruguay.

As the two Uruguayan geologists worked to analyze and describe the different rocks, Pecoits noticed the lines in the stone. It was a chance discovery, but "he knew he was onto a winner," Konhauser said.

The team eventually found the trails, extending up to 40 centimetres in length, in six different locations.

From the ridges left behind along the burrows, the researchers believe the unnamed slug, known as a bilaterian because of its symmetrical sides, moved through the sands with wavelike movements. Indents along the trail show the slug had primitive feet of sorts that could help it move.

At different points in the trail, researchers said it appears the rice grain-sized slug would abruptly leave. They speculate that it was surfacing from the sands below the ocean floor into the bacterial slime above to eat or to get oxygen.

"From an evolutionary point of view, this is quite complex behaviour," Pecoits said. "Once the organism starts moving, that's a huge stage of evolution."

To put this discovery in perspective, it helps to have a sense of where it fits into the geological time scale. The Earth is about 4.6 billion years old. Single-cell life forms, starting with bacteria, began to appear about 3.5 billion years ago. Other studies have shown that creatures like sponges appeared about 700 million years ago. By comparison, the earliest dinosaurs are relative newcomers, first appearing in the fossil record about 245 million years ago.

Prior to finding these slug burrows, researchers say the oldest confirmed discovery of a creature that could move dated back 555 million years ago.

Like the U of A's discovery in Uruguay, that 555 million-year-old find in Russia was a trace fossil, which means that scientists see evidence of the animal's movements such as footprints or burrows, not the animal itself.

Those marks are preserved as layer after layer of sediment pile on top of it, eventually become so compact that it goes through a chemical conversion and turns to rock.

But to make a 30 million year leap backward was a thrill for the U of A scientists and colleagues at the Universidad de la Republica in Uruguay.

"To move in one foul swoop 30 million years is significant," Konhauser said.

For one, the researchers say, it shows that life evolved quite quickly — geologically speaking — going from sponges to these moving slugs in just tens of millions of years.

In addition to finding the trails now exposed in the rock, scientists say there was an equally important second discovery that allowed them to date the find.

Interviewed by phone this week as they attended a geochemistry conference in Montreal, Pecoits, Aubet and Konhauser explained that granite was intruding into the siltstone that held the trace fossils. It is very difficult to date the age of a sedimentary rock. But geologists can determine the age of granite, a volcanic rock, since it contains a mineral called zircon and zircon reveals its age by its radioactive decay.

Pecoits, Aubet and Konhauser called on colleagues at the U of A who specialize in pinpointing the age of rocks and identifying trace fossils.

"We weren't sure the traces were so old, but after dating the granite, that was the very first big surprise that we had," Pecoits said.

The first test of the rocks showed them to be 585 million years old, which meant the slug trails were at least that old. And that was just the start of more than four years of testing, field work back at the site in Uruguay, writing and questioning from a peer review panel.

The researchers said they know their findings will be scrutinized. "Someone is going to look at this and try to disprove it," Konhauser said. "That's the way science works."

But Konhauser, Pecoits and Aubet said they are confident that almost five years worth of study and testing will hold up. While all three are at different stages in their careers, they all agreed it is their most exciting find to date.

"For me, it is the biggest discovery ever," Pecoits said.

"I'm still a PhD student," Aubet said. "So I can tell you, this is perfect."

sodonnell@edmontonjournal.com

Twitter.com/scodonnell

Edmonton Journal

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Alberta researchers help find earliest evidence of mobile life on Earth

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